This list is FAR from inclusive. If you want to see everything available on the web to help teachers, there are plenty of places to look. Web 2.0/21st Century Tools (a wiki put together by David Kapuler) is one that includes an exhaustive list of tools, and what they do. This list contains tools that I have personally used, and have found easy to use, and that I think other teachers will find useful. If you know of a tool that meets these criteria (you've used it, it's easy to use, and useful for teachers) please let me know about it so that I can add it to this list :)